In the following, vehicle brake systems having slip regulation are understood as hydraulic brake systems having anti-lock protection (ABS), anti-slip regulation (ASR), and/or driving dynamics regulation (ESP), in which the brake pressure of the wheel brakes is set as a function of the slippage conditions at each of the associated wheels. This pressure adaptation can be carried out automatically, i.e. without driver involvement, or with involvement of the driver. The pressure medium units used for this purpose have, inter alia, a housing block equipped with pressure producers, storage units, and control valves, in which block a multiplicity of pressure medium-conducting ducts are configured in order to connect the named components, or the receptacles formed in the housing block for these components, with one another so as to conduct pressure medium. The control valves are electronically controllable and permit modification of the cross-section of an associated duct in order to control pressure as needed. For this purpose, an electronic control device is present that carries out an adapted electronic controlling of the control valves as a function of the brake pressure to be set. In addition, the electronic control device also controls a motor, attached to the housing block, that actuates the pressure producers.
Contaminants that may be present in the pressure medium can cause damage to the components that control the pressure medium, in particular the control valves, and can result in undesirable leaks. In order to avoid these disadvantages, it is known from the existing art to equip pressure medium units with filter elements.
FIG. 1 shows a pressure medium unit 10 of applicant, known from the existing art, having a housing block 12, a control valve 14 situated on this housing block 12, and a filter element 16 that is situated in the interior of a pressure medium-conducting duct 18, also configured on housing block 12. An axial filter 20 is assigned to control valve 14 in order to filter contaminants out from the pressure medium before this pressure medium enters into control valve 14. For this purpose, axial filter 20 is for example anchored at a position at which a second duct 22 opens into first duct 18. Axial filter 20 is made up of a hollow cylindrical filter framework, having openings made on its circumference. Pressure medium flows through these openings, and the openings are covered by a filter mesh having a defined mesh size. Pressure medium flows out from second duct 22 through the mesh weave of the filter mesh, into the hollow interior of axial filter 20, where it is diverted and, in the filtered state, flows through the open end of axial filter 20, facing control valve 14. With its second end, facing away from control valve 14, the axial filter is supported on the base of first duct 18, configured as a blind hole.
The mentioned filter framework is standardly made of plastic, and can be injection-molded onto the filter mesh. It lends stability to the filter mesh, which is flexible in itself, so that axial filter 20 can be introduced into the pressure medium-conducting first duct 18 with radial pre-tension, and can be placed therein. Accordingly, a non-positive connection exists between axial filter 20 and the wall of duct 18, by which axial filter 20 is fixed circumferentially in a sealing manner in duct 18. In the depicted case, an axial fixing of axial filter 20 in first duct 18 takes place by pressing in axial filter 20 until it contacts the base of first duct 18, and through the subsequent mounting of control valve 14, which in the assembled state lies against the first end, at the outflow side, of axial filter 20 with a valve connecting piece that extends in the direction of assembly. Because the spacing between the end face of the valve connecting piece and the base of first duct 18 is standardly somewhat smaller than the length of axial filter 20, in the assembled state additional axial pre-tension forces act on axial filter 20.
It is known that as a result of changes in temperature, components made of different materials expand by different amounts, or contract by different amounts when there is a corresponding cooling. An axial filter 20 made essentially of plastic expands more, given increasing temperatures, than does the surrounding housing block 12 or control valve 14, which are both made predominantly of metal. In the case of, for example, decreasing temperatures, this can result in the occurrence of axial and/or radial play at axial filter 20, or axial filter 20 can execute small movements in the interior of first duct 18. In the extreme case, a hydraulic bypass of axial filter 20 may even arise, through which unfiltered pressure medium can penetrate to control valve 14, where it can cause damage or leaks.